The female genital tract
The fertilization process takes place in the female genital tract.
The ovary and the dominant follicle
Roughly a week before the midpoint of the menstrual cycle the dominant follicle develops in one of the two ovaries. This grows faster than the other tertiary follicles and prepares itself for ovulation. It reaches a diameter of up to 25 mm and is also known then as the graafian follicle.
The ultrasound image in Fig. 7 shows the follicle fluid as a black surface. The recording was made shortly before ovulation. Since at this moment the cumulus oophorus has detached itself from the granulosa, the oocyte "swims" surrounded by its cumulus cells (so-called corona radiata) in the follicle fluid. This cannot be seen, though, in the figure.
The graafian follicle bulges out the ovary's surface. It is ready to tear open, i.e., its wall and the peritoneal covering will soon rupture and its watery contents will pour into the infundibulum of the tuba, together with the oocyte. The diameter of the follicle that is about to rupture is here 22 mm (corresponding to 5 ml of follicle fluid).
A hormonal regulation is responsible for follicle maturation. The selection criteria for choosing the dominant follicle are still unknown, however. The following plot shows the course of the concentrations of the hormones that are involved during a monthly cycle.
Approximately one-and-a-half days before the midpoint of the cycle, the concentration of the luteinizing hormone (LH) rises steeply. The LH peak is responsible for a whole series of processes: triggering ovulation, creating an optimal milieu for a successful fertilization, and harmonizing the temporal sequence of the processes with one another.
With the LH peak the following maturation steps are now triggered in and around the oocyte - up to ovulation:
In the oocyte:
- Termination of the first meiosis with ejection of the first polar body
- Begin of the second meiosis with arrest in the metaphase
- Maturation of the oocyte cytoplasma by preparing molecules and structures that will be needed at the time of fertilization
In the follicle:
- the granulosa cells that sit just outside on the pellucid zone withdraw their processes from the oocyte surface back into the pellucid zone. Those processes were in charge of transferring substances to the oocyte
- The perivitelline space forms between the oocyte and the pellucid zone. This space is necessary for allowing division of the oocyte and for harboring the first polar body formed in the division.
- Loosening of the granulosa cells in the vicinity of the cumulus oophorus and proliferation of the granulosa cells
- Increasing the progesterone concentration in the follicle fluid via increased production in the granulosa cells
The results of these processes are:
- The correct placement of the uterine tube infundibulum upon the ovarian surface
- The rupture of the follicle wall and the flow of the follicle fluid with the oocyte into the infundibulum
- The inhibition of the maturation of further follicles
More detail about these effects is provided in the next pages.